Metabolic Changes Associated with the Germination of Corn. III. Effects of Gibberellic Acid on Endosperm Metabolism

Stimulation of endosperm metabolism by the addition of exogenous gibberellic acid (GA) has been reported by many workers. A series of papers by Paleg (8, 9, 10) have described the dependence of loss of dry weight, starch hydrolysis and protein release in excised barlev endosperm on the presence of added GA. Work with many varieties of barley, wheat and oats has confirmed the generality of this effect (11, 12). Since GA-like compounds occur in barley embryos (5) and in many higher-plant tissues (1, 7), it is likely that endosperm metabolism is initiated by the release of such compounds from the embryo during normal generation. More detailed studies have been made on the GA stimulation of a-amylase activity in barley endosperm in an attempt to elucidate the mechanism of action. Paleg (10) has suggested that the hormone activates or releases proteases, which in turn activate or release the a-amylase; whereas MacLeod and Millar (6) proposed that GA brought about the release of hydrolytic enzymes, including a-amylase from lysosome-like particles. Recently, however, Briggs (2) has reported an enhanced de novo synthesis of proteins in the aleurone cells of barley endosperm in response to GA, and Varner (13) has demonstrated the stimulation of de novo synthesis of a-amylase in the same tissue. This paper reports the results of experiments designed to test whether carbohydrate and protein catabolism in excised corn endosperm is dependent on exogenous GA as in other cereal grains. In addition, the effects of GA on the activities of ribonuclease and 3'nucleotidase in the endosperm were examined. These 2 enzymes are invo!ved in nucleic acid catabolism, and their activities increased markedly during germination (4).